In recent years, a variety of mobile devices, starting with cellular telephones, have become widespread. At the same time, development of displays used in these devices has progressed rapidly. In addition to liquid crystal displays (LCDs) and organic electroluminescent displays (OLEDs), electronic paper has also emerged on the market.
These display devices include a display panel having pixels arranged in a matrix on a substrate, the pixels being composed of display elements such as EL elements, liquid crystal elements, electronic paper, or the like. The display devices also include a driving means that applies voltage to the display elements. Display characteristics differ depending on the type of display elements forming the pixels.
For example, OLEDs emit light and allow for high-performance, full-color video display. Like transmissive type LCDs, however, OLEDs exhibit reduced brightness and contrast in direct sunlight, making it difficult to achieve satisfactory image display. While the luminous efficiency of OLEDs has improved over recent years, considerable power consumption is required even when displaying black and white images, such as electronic books.
On the other hand, electronic paper is a reflective display device that has been used in recent years in devices such as electronic book terminals. Types of electronic paper include electrophoretic and cholesteric liquid crystal types.
For example, in the panel disclosed in Patent Literature 2, a front substrate and a back substrate are disposed to face each other with a space therebetween. Partition walls are provided in the space to partition adjacent cells, and colored particles are injected in each partitioned cell space. Voltage is applied across a cell electrode along the back substrate in each cell space and a focusing electrode provided between adjacent cell spaces, thereby causing the colored particles to move within the cell spaces.
Since this electronic paper retains images, display can be maintained without consuming nearly any electricity until the display screen is changed. Paper-like display devices such as electronic paper are chiefly black and white, but color displays are also being developed.
For example, one type of display device achieves color display by attaching a color filter to each cell of the display device in front of the cell space. In this type, however, the light reflected from each cell space is absorbed by the color filter, thus lowering the degree of reflection.
By contrast, Patent Literature 3 discloses an electrophoretic display device that displays images by providing two layered sets of a display electrode and a focusing electrode in each cell space, injecting translucent colored particles of two colors selected from among cyan, yellow, magenta, black, and the like, and causing the particles to migrate between the display electrode and the focusing electrode.
This display device can display a plurality of colors in each pixel without using a color filter by collecting colored particles necessary for color display on the display electrode and shielding light from colored particles not necessary for display by causing these particles to attach to the back side of the focusing electrode. While this yields a color display with bright, sharp colors, many problems remain to be resolved, such as improvement in response speed, assurance of gradation, improvement in degree of reflection, and more.